The affix ṭac comes after go standing at the end of a tat-puruṣa samāsa when no elision of a taddhita-affix has taken place in forming such a samāsa ,
Thus परमगवः, उत्तमगवः, पंचगवम्, दशगवम् ॥ Why do we say \when no Taddhita affix has been elided in forming the compound?\ Observe, पंचभिर्गोभिः क्रीतः = पंचगुः (Here the affix ठक् 5.1.37 is elided by 5.1.28 and hence the samasanta affix टच् is not added). So also दशगुः ॥
Why do we say \when a Taddhita is elided\? The prohibition does not apply when a सुप् affix is elided : as राजगवमिच्छति = राजगवीयति 3.1.8 and 2.4.71.
Why do we say 'when there is elision' ? Merely adding a Taddhita affix in forming a Tatpurusha, will not debar the samasanta affix: as पंचभ्यो गोभ्य आगतं = पंचगो + टच् + रुप्य (the tach is added by this sutra, and the taddhita रुप्य to denote तत आगतः by 4.3.81) = पंचगवरुप्यम्, पंचगवमयम्, दशगवरुप्यम् दशगवमयम् ॥,